Injection Molding Machines and Methods for Accounting for Changes in Material Properties During Injection Molding Runs

ABSTRACT

A method and a machine account for changes in material properties of molten plastic material during an injection run. A change in a control signal is calculated by a controller during the injection molding run. If the change in the control signal indicates a change in material flowability, the controller alters a target injection pressure to ensure that molten plastic material completely fills and packs a mold cavity to prevent part flaws such as short shots or flashing.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to injection molding machines and methodsof producing injection molded parts and, more particularly, to injectionmolding machines that adjust operating parameters of the injectionmolding machine during an injection molding run to account for changesin material properties of the injection material and methods ofaccounting for changes in injection molding material properties duringan injection molding run.

BACKGROUND OF THE INVENTION

Injection molding is a technology commonly used for high-volumemanufacturing of parts made of meltable material, most commonly of partsmade of thermoplastic polymers. During a repetitive injection moldingprocess, a plastic resin, most often in the form of small beads orpellets, is introduced to an injection molding machine that melts theresin beads under heat, pressure, and shear. The now molten resin isforcefully injected into a mold cavity having a particular cavity shape.The injected plastic is held under pressure in the mold cavity, cooled,and then removed as a solidified part having a shape that essentiallyduplicates the cavity shape of the mold. The mold itself may have asingle cavity or multiple cavities. Each cavity may be connected to aflow channel by a gate, which directs the flow of the molten resin intothe cavity. A molded part may have one or more gates. It is common forlarge parts to have two, three, or more gates to reduce the flowdistance the polymer must travel to fill the molded part. The one ormultiple gates per cavity may be located anywhere on the part geometry,and possess any cross-section shape such as being essentially circularor be shaped with an aspect ratio of 1.1 or greater. Thus, a typicalinjection molding procedure comprises four basic operations: (1) heatingthe plastic in the injection molding machine to allow the plastic toflow under pressure; (2) injecting the melted plastic into a mold cavityor cavities defined between two mold halves that have been closed; (3)allowing the plastic to cool and harden in the cavity or cavities whileunder pressure; and (4) opening the mold halves and ejecting the partfrom the mold.

During the injection molding process, the molten plastic resin isinjected into the mold cavity and the plastic resin is forcibly injectedinto the cavity by the injection molding machine until the plastic resinreaches the location in the cavity furthest from the gate. Thereafter,the plastic resin fills the cavity from the end back towards the gate.The resulting length and wall thickness of the part is a result of theshape of the mold cavity.

In some cases, it may be desirous to reduce the wall thickness ofinjected molded parts to reduce the plastic content, and thus cost, ofthe final part. Reducing wall thickness using a conventional highvariable pressure injection molding process can be an expensive and anon-trivial task. In fact, conventional injection molding machines (e.g.machines injecting molten plastic resin between about 8,000 psi andabout 20,000 psi) have a practical limit as to how thin walls of a partmay be molded. Generally speaking, conventional injection moldingmachines cannot mold parts having a thinwall ratio (as defined by an L/Tratio set forth below) of greater than about 200. Furthermore, moldingthinwall parts with thinwall ratios of more than 100 requires pressuresat the high end of current capability and thus, presses that are capableof handling these high pressures.

When filling a thinwall part, the current industry practice is to fillthe mold cavity at the highest possible rate the molding machine canachieve. This approach ensures that the mold cavity is filled before thepolymer solidifies or “freezes off” in the mold, and provides the lowestpossible cycle time since the polymer is exposed to the cooled moldcavity as quickly as possible. This approach has two drawbacks. Thefirst is that to achieve very high filling velocities requires very highpower loads, and this requires very expensive molding equipment.Further, most electric presses are unable to provide sufficient power toachieve these high filling rates, or require very complicated andexpensive drive systems that substantially increase the cost of themolding equipment making them impractical economically.

The second drawback is that the high filling rates require very highpressures. These high pressures result in the need for very highclamping forces to hold the mold closed during filling, and these highclamping forces result in very expensive molding equipment. The highpressures also require injection mold cores that are made from very highstrength materials, typically hardened tool steels. These high strengthmolds are also very expensive, and can be impractical economically formany molded components. Even with these substantial drawbacks, the needfor thinwall injection molded components remains high, since thesecomponents use less polymer material to form the molded part, therebyresulting in material savings that more than offset the higher equipmentcosts. Further, some molded components require very thin design elementsto perform properly, such as design elements that need to flex, ordesign elements that must mate with very small features of other designelements.

As a liquid plastic resin is introduced into an injection mold in aconventional injection molding process the material adjacent to thewalls of the cavity, immediately begins to “freeze,” or solidify, orcure, or in the case of crystalline polymers the plastic resin begins tocrystallize, because the liquid plastic resin cools to a temperaturebelow the material's no flow temperature and portions of the liquidplastic become stationary. This frozen material adjacent to the walls ofthe mold narrows the flow path that the thermoplastic travels as itprogresses to the end of the mold cavity. The thickness of the frozenmaterial layer adjacent to the walls of the mold increases as thefilling of the mold cavity progresses, this causes a progressivereduction in the cross sectional area the polymer must flow through tocontinue to fill the mold cavity. As material freezes, it also shrinks,pulling away from the mold cavity walls, which reduces effective coolingof the material by the mold cavity walls. As a result, conventionalinjection molding machines fill the mold cavity with plastic veryquickly and then maintain a packing pressure to force the materialoutward against the sides of the mold cavity to enhance cooling and tomaintain the correct shape of the molded part. Conventional injectionmolding machines typically have cycle times made up of about 10%injection time, about 50% packing time, and about 40% cooling time.

As plastic freezes in the mold cavity, conventional injection moldingmachines increase injection pressure (to maintain a substantiallyconstant volumetric flow rate due to the smaller cross-sectional flowarea). Increasing the pressure, however, has both cost and performancedownsides. As the pressure required to mold the component increases, themolding equipment must be strong enough to withstand the additionalpressure, which generally equates to being more expensive. Amanufacturer may have to purchase new equipment to accommodate theseincreased pressures. Thus, a decrease in the wall thickness of a givenpart can result in significant capital expenses to accomplish themanufacturing via conventional injection molding techniques.

In an effort to avoid some of the drawbacks mentioned above, manyconventional injection molding operations use shear-thinning plasticmaterial to improve flow characteristics of the plastic material intothe mold cavity. As the shear-thinning plastic material is injected intothe mold cavity, shear forces generated between the plastic material andthe mold cavity walls tend to reduce viscosity of the plastic material,thereby allowing the plastic material to flow more freely and easilyinto the mold cavity. As a result, it is possible to fill thinwall partsfast enough to avoid the material completely freezing off before themold is completely filled.

Reduction in viscosity is directly related to the magnitude of shearforces generated between the plastic material and the feed system, andbetween the plastic material and the mold cavity wall. Thus,manufacturers of these shear-thinning materials and operators ofinjection molding systems have been driving injection molding pressureshigher in an effort to increase shear, thus reducing viscosity.Typically, high output injection molding systems (e.g., class 101 andclass 30 systems) inject the plastic material in to the mold cavity atmelt pressures of typically 15,000 psi or more. Manufacturers ofshear-thinning plastic material teach injection molding operators toinject the plastic material into the mold cavities above a minimum meltpressure. For example, polypropylene resin is typically processed atpressures greater than 6,000 psi (the recommended range from thepolypropylene resin manufacturers, is typically from greater than 6,000psi to about 15,000 psi). Press manufacturers and processing engineerstypically recommend processing shear thinning polymers at the top end ofthe range, or significantly higher, to achieve maximum potential shearthinning, which is typically greater than 15,000 psi, to extract maximumthinning and better flow properties from the plastic material. Shearthinning thermoplastic polymers generally are processed in the range ofover 6,000 psi to about 30,000 psi. Even with the use of shear thinningplastics, a practical limit exists for high variable pressure injectionmolding of thin walled parts. This limit is currently in the range ofthinwall parts having a thinwall ratio of 200 or more. Moreover, evenparts having a thinwall ratio of between 100 and 200 may become costprohibitive as these parts generally require injection pressures betweenabout 15,000 psi and about 20,000 psi.

High production injection molding machines (i.e., class 101 and class 30molding machines) that produce thinwalled consumer products exclusivelyuse molds having a majority of the mold made from high hardnessmaterials. High production injection molding machines typicallyexperience 500,000 cycles per year or more. Industrial qualityproduction molds must be designed to withstand at least 500,000 cyclesper year, preferably more than 1,000,000 cycles per year, morepreferably more than 5,000,000 cycles per year, and even more preferablymore than 10,000,000 cycles per year. These machines have multi cavitymolds and complex cooling systems to increase production rates. The highhardness materials are more capable of withstanding the repeated highpressure clamping operations than lower hardness materials. However,high hardness materials, such as most tool steels, have relatively lowthermal conductivities, generally less than 20 BTU/HR FT ° F., whichleads to long cooling times as heat is transferred through from themolten plastic material through the high hardness material.

Even with the ever increasing injection pressure ranges of existing highvariable pressure injection molding machines, a practical limit remainsof about 200 (L/T ratio) for molding thinwalled parts in conventionalhigh (e.g., 20,000 psi) variable pressure injection molding machines andthinwall parts having a thinwall ratio of between about 100 and about200 may be cost prohibitive for many manufacturers.

Changes in molding conditions can significantly affect properties of themolten plastic material. More specifically, changes in environmentalconditions (such as changes in temperature) can raise or lower theviscosity of the molten plastic material. When viscosity of the moltenplastic material changes, quality of the molded part may be impacted.For example, if the viscosity of the molten plastic material increasesthe molded part may experience a short shot, or a shortage of moltenplastic material. On the other hand, if the viscosity of the moltenplastic material decreases the molded part may experience flashing asthe thinner molten plastic material is pressed into the seam of the moldcavity. Furthermore, recycled plastic material that is mixed with virginmaterial may change a melt flow index (MFI) of the combined plasticmaterial. Conventional injection molding machines do not adjustoperating parameters to account for these changes in materialproperties. As a result, conventional injection molding machines canproduce lower quality parts, which must be removed duringquality-control inspections, thereby leading to operationalinefficiencies.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments set forth in the drawings are illustrative and exemplaryin nature and not intended to limit the subject matter defined by theclaims. The following detailed description of the illustrativeembodiments can be understood when read in conjunction with thefollowing drawings, where like structure is indicated with likereference numerals and in which:

FIG. 1 illustrates a schematic view of an injection molding machineconstructed according to the disclosure;

FIG. 2 illustrates one embodiment of a thin-walled part formed in theinjection molding machine of FIG. 1;

FIG. 3 is a cavity pressure vs. time graph for the injection moldingmachine of FIG. 1 superimposed over a cavity pressure vs. time graph fora conventional injection molding machine;

FIG. 4 is another cavity pressure vs. time graph for the injectionmolding machine of FIG. 1 superimposed over a cavity pressure vs. timegraph for a conventional injection molding machine, the graphsillustrating the percentage of fill time devoted to certain fill steps;

FIGS. 5A-5D are side cross-sectional views of a portion of a thinwallmold cavity in various stages of fill by a conventional injectionmolding machine;

FIGS. 6A-6D are side cross-sectional views of a portion of a thinwallmold cavity in various stages of fill by the injection molding machineof FIG. 1;

FIG. 7 is a schematic illustration of an injection molding cycle thatmay be carried out on the injection molding machine of FIG. 1;

FIG. 8 is a pressure vs. time graph for an injection molding machinethat illustrates the effect of variations in viscosity of the moltenplastic material;

FIG. 9A is a graph that illustrates changes in control signal voltageover time during an injection molding cycle;

FIG. 9B is a graph that illustrates changes in control signal voltageover a distance traveled for a melt moving machine component;

FIG. 10 is a logic diagram that illustrates an injection molding processthat accounts for viscosity changes in the molten plastic material; and

FIG. 11 is a schematic diagram of a control system that may be used toimplement the logic diagram of FIG. 10.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention generally relate to systems,machines, products, and methods of producing products by injectionmolding and more specifically to systems, products, and methods ofproducing products by low substantially constant pressure injectionmolding. However, the devices and methods for accounting for viscositychanges in the molten plastic material described herein are not limitedto low substantially constant pressure injection molding machines andprocesses. Rather, the disclosed devices and methods for accounting forviscosity changes in the molten plastic material may be incorporatedinto virtually any injection molding machine or process, including, butnot limited to, high pressure processes, low pressure processes,variable pressure processes, and constant or substantially constantpressure processes.

The term “low pressure” as used herein with respect to melt pressure ofa thermoplastic material, means melt pressures in a vicinity of a nozzleof an injection molding machine of 15,000 psi and lower.

The term “substantially constant pressure” as used herein with respectto a melt pressure of a thermoplastic material, means that deviationsfrom a baseline melt pressure do not produce meaningful changes inphysical properties of the thermoplastic material. For example,“substantially constant pressure’ includes, but is not limited to,pressure variations for which viscosity of the melted thermoplasticmaterial do not meaningfully change. The term “substantially constant”in this respect includes deviations of approximately 30% from a baselinemelt pressure. For example, the term “a substantially constant pressureof approximately 4600 psi” includes pressure fluctuations within therange of about 6000 psi (30% above 4600 psi) to about 3200 psi (30%below 4600 psi). A melt pressure is considered substantially constant aslong as the melt pressure fluctuates no more than 30% from the recitedpressure.

The term “melt holder”, as used herein, refers to the portion of aninjection molding machine that contains molten plastic in fluidcommunication with the machine nozzle. The melt holder is heated, suchthat a polymer may be prepared and held at a desired temperature. Themelt holder is connected to a power source, for example a hydrauliccylinder or electric servo motor, that is in communication with acentral control unit, and can be controlled to advance a diaphragm toforce molten plastic through the machine nozzle. The molten materialthen flows through the runner system in to the mold cavity. The meltholder may be cylindrical in cross section, or have alternative crosssections that will permit a diaphragm to force polymer under pressuresthat can range from as low as 100 psi to pressures 40,000 psi or higherthrough the machine nozzle. The diaphragm may optionally be integrallyconnected to a reciprocating screw with flights designed to plasticizepolymer material prior to injection.

The term “high L/T ratio” generally refers to L/T ratios of 100 orgreater, and more specifically to L/T ratios of 200 or greater, but lessthan 1000. Calculation of the L/T ratio is defined below. The term “peakflow rate” generally refers to the maximum volumetric flow rate, asmeasured at the machine nozzle.

The term “peak injection rate” generally refers to the maximum linearspeed the injection ram travels in the process of forcing polymer in tothe feed system. The ram can be a reciprocating screw such as in thecase of a single stage injection system, or a hydraulic ram such as inthe case of a two stage injection system.

The term “ram rate” generally refers to the linear speed the injectionram travels in the process of forcing polymer into the feed system.

The term “flow rate” generally refers to the volumetric flow rate ofpolymer as measured at the machine nozzle. This flow rate can becalculated based on the ram rate and ram cross sectional area, ormeasured with a suitable sensor located in the machine nozzle.

The term “cavity percent fill” generally refers to the percentage of thecavity that is filled on a volumetric basis. For example, if a cavity is95% filled, then the total volume of the mold cavity that is filled is95% of the total volumetric capacity of the mold cavity.

The term “melt temperature” generally refers to the temperature of thepolymer that is maintained in the melt holder, and in the material feedsystem when a hot runner system is used, which keeps the polymer in amolten state. The melt temperature varies by material, however, adesired melt temperature is generally understood to fall within theranges recommended by the material manufacturer.

The term “gate size” generally refers to the cross sectional area of agate, which is formed by the intersection of the runner and the moldcavity. For hot runner systems, the gate can be of an open design wherethere is no positive shut off of the flow of material at the gate, or aclosed design where a valve pin is used to mechanically shut off theflow of material through the gate in to the mold cavity (commonlyreferred to as a valve gate). The gate size refers to the crosssectional area, for example a 1 mm gate diameter refers to a crosssectional area of the gate that is equivalent to the cross sectionalarea of a gate having a 1 mm diameter at the point the gate meets themold cavity. The cross section of the gate may be of any desired shape.

The term “effective gate area” generally refers to a cross sectionalarea of a gate corresponding to an intersection of the mold cavity and amaterial flow channel of a feed system (e.g., a runner) feedingthermoplastic to the mold cavity. The gate could be heated or notheated. The gate could be round, or any cross sectional shape, suited toachieve the desired thermoplastic flow into the mold cavity. The term“intensification ratio” generally refers to the mechanical advantage theinjection power source has on the injection ram forcing the moltenpolymer through the machine nozzle. For hydraulic power sources, it iscommon that the hydraulic piston will have a 10:1 mechanical advantageover the injection ram. However, the mechanical advantage can range fromratios much lower, such as 2:1, to much higher mechanical advantageratio such as 50:1.

The term “peak power” generally refers to the maximum power generatedwhen filling a mold cavity. The peak power may occur at any point in thefilling cycle. The peak power is determined by the product of theplastic pressure as measured at the machine nozzle multiplied by theflow rate as measured at the machine nozzle. Power is calculated by theformula P=p*Q where p is pressure and Q is volumetric flow rate.

The term “volumetric flow rate” generally refers to the flow rate asmeasured at the machine nozzle. This flow rate can be calculated basedon the ram rate and ram cross sectional area, or measured with asuitable sensor located in the machine nozzle.

The terms “filled” and “full,” when used with respect to a mold cavityincluding thermoplastic material, are interchangeable and both termsmean that thermoplastic material has stopped flowing into the moldcavity.

The term “shot size” generally refers to the volume of polymer to beinjected from the melt holder to completely fill the mold cavity orcavities. The Shot Size volume is determined based on the temperatureand pressure of the polymer in the melt holder just prior to injection.In other words, the shot size is a total volume of molten plasticmaterial that is injected in a stroke of an injection molding ram at agiven temperature and pressure. Shot size may include injecting moltenplastic material into one or more injection cavities through one or moregates. The shot of molten plastic material may also be prepared andinjected by one or more melt holders.

The term “hesitation” generally refers to the point at which thevelocity of the flow front is minimized sufficiently to allow a portionof the polymer to drop below its no flow temperature and begin to freezeoff.

The term “electric motor” or “electric press,” when used herein includesboth electric servo motors and electric linear motors.

The term “Peak Power Flow Factor” refers to a normalized measure of peakpower required by an injection molding system during a single injectionmolding cycle and the Peak Power Flow Factor may be used to directlycompare power requirements of different injection molding systems. ThePeak Power Flow Factor is calculated by first determining the PeakPower, which corresponds to the maximum product of molding pressuremultiplied by flow rate during the filling cycle (as defined herein),and then determining the Shot Size for the mold cavities to be filled.The Peak Power Flow Factor is then calculated by dividing the Peak Powerby the Shot Size.

The term “low constant pressure injection molding machine” is defined asa class 101 or a class 30 injection molding machine that uses asubstantially constant injection pressure that is less than 15,000 psi.Alternatively, the term “low constant pressure injection moldingmachine” may be defined as an injection molding machine that uses asubstantially constant injection pressure that is less than 15,000 psiand that is capable of performing more than 1 million cycles, preferablymore than 1.25 million cycles, more preferably more than 2 millioncycles, more preferably more than 5 million cycles, and even morepreferably more than 10 million cycles before the mold core (which ismade up of first and second mold parts that define a mold cavitytherebetween) reaches the end of its useful life. Characteristics of“low constant pressure injection molding machines” include mold cavitieshaving an L/T ratio of greater than 100 (and preferably greater than200), multiple mold cavities (preferably 4 mold cavities, morepreferably 16 mold cavities, more preferably 32 mold cavities, morepreferably 64 mold cavities, more preferably 128 mold cavities and morepreferably 256 mold cavities, or any number of mold cavities between 4and 512), a heated runner, and a guided ejection mechanism.

The term “useful life” is defined as the expected life of a mold partbefore failure or scheduled replacement. When used in conjunction with amold part or a mold core (or any part of the mold that defines the moldcavity), the term “useful life” means the time a mold part or mold coreis expected to be in service before quality problems develop in themolded part, before problems develop with the integrity of the mold part(e.g., galling, deformation of parting line, deformation or excessivewear of shut-off surfaces), or before mechanical failure (e.g., fatiguefailure or fatigue cracks) occurs in the mold part. Typically, the moldpart has reached the end of its “useful life” when the contact surfacesthat define the mold cavity must be discarded or replaced. The moldparts may require repair or refurbishment from time to time over the“useful life” of a mold part and this repair or refurbishment does notrequire the complete replacement of the mold part to achieve acceptablemolded part quality and molding efficiency. Furthermore, it is possiblefor damage to occur to a mold part that is unrelated to the normaloperation of the mold part, such as a part not being properly removedfrom the mold and the mold being force ably closed on the non-ejectedpart, or an operator using the wrong tool to remove a molded part anddamaging a mold component. For this reason, spare mold parts aresometimes used to replace these damaged components prior to themreaching the end of their useful life. Replacing mold parts because ofdamage does not change the expected useful life.

The term “guided ejection mechanism” is defined as a dynamic part thatactuates to physically eject a molded part from the mold cavity.

The term “coating” is defined as a layer of material less than 0.13 mm(0.005 in) in thickness, that is disposed on a surface of a mold partdefining the mold cavity, that has a primary function other thandefining a shape of the mold cavity (e.g., a function of protecting thematerial defining the mold cavity, or a function of reducing frictionbetween a molded part and a mold cavity wall to enhance removal of themolded part from the mold cavity).

The term “average thermal conductivity” is defined as the thermalconductivity of any materials that make up the mold cavity or the moldside or mold part. Materials that make up coatings, stack plates,support plates, and gates or runners, whether integral with the moldcavity or separate from the mold cavity, are not included in the averagethermal conductivity. Average thermal conductivity is calculated on avolume weighted basis.

The term “effective cooling surface” is defined as a surface throughwhich heat is removed from a mold part. One example of an effectivecooling surface is a surface that defines a channel for cooling fluidfrom an active cooling system. Another example of an effective coolingsurface is an outer surface of a mold part through which heat dissipatesto the atmosphere. A mold part may have more than one effective coolingsurface and thus may have a unique average thermal conductivity betweenthe mold cavity surface and each effective cooling surface.

The term “nominal wall thickness” is defined as the theoreticalthickness of a mold cavity if the mold cavity were made to have auniform thickness. The nominal wall thickness may be approximated by theaverage wall thickness. The nominal wall thickness may be calculated byintegrating length and width of the mold cavity that is filled by anindividual gate.

The term “average hardness” is defined as the Rockwell hardness for anymaterial or combination of materials in a desired volume. When more thanone material is present, the average hardness is based on a volumeweighted percentage of each material. Average hardness calculationsinclude hardnesses for materials that make up any portion of the moldcavity. Average hardness calculations do not include materials that makeup coatings, stack plates, gates or runners, whether integral with amold cavity or not, and support plates. Generally, average hardnessrefers to the volume weighted hardness of material in the mold coolingregion.

The term “mold cooling region” is defined as a volume of material thatlies between the mold cavity surface and an effective cooling surface.

The term “cycle time” or “injection molding cycle” is defined as asingle iteration of an injection molding process that is required tofully form an injection molded part. Cycle time, or injection moldingcycle, includes the steps of advancing molten thermoplastic materialinto a mold cavity, substantially filling the mold cavity withthermoplastic material, cooling the thermoplastic material, separatingfirst and second mold sides to expose the cooled thermoplastic material,removing the thermoplastic material, and closing the first and secondmold sides.

The term “injection molding run,” as used herein, includes a series ofsequential injection molding cycles that are performed on a commoninjection molding machine.

The term “flowability,” as used herein, includes the flow resistance ofa molten plastic material as it flows through an injection moldingsystem and accounts for all influences on the relative viscosity of themolten plastic material, including, but not limited to, composition ofthe molten plastic material, temperature, shear, mold design, and partdesign.

The term “flow factor” is defined as a ratio of a control signal for aproportional valve to an incremental time period. This ratio may beexpressed in voltage/time, for example, in millivolts/microsecond. Thisratio may be determined for any type of injection press (e.g., ahydraulic press or an electric press) and may be calculated by theformula: FF=(CS1−CS2)/T, where CS1 and CS2 are control signals measuredover an incremental time period.

The term “viscosity change index” is defined as a ratio of a controlsignal for a proportional valve over a given distance of travel for amelt moving machine component, such as an injection screw, in aninjection molding machine. This ratio may be expressed involtage/distance, for example, in millivolts/micron. This ratio may bedetermined for any type of injection press (e.g., a hydraulic press oran electric press) and may be calculated by the formula:VCI=(CS1−CS2)/S, where CS1 is a first control signal, CS2 is a secondcontrol signal, and S is a distance of travel for a melt moving machinecomponent.

The term “proportional valve” is defined as a valve that moves inproportion to an electronic control signal. For example, if anelectronic control signal increases by 10%, the proportional valve willopen to allow 10% more material to flow through the valve.

Low constant pressure injection molding machines may also be highproductivity injection molding machines (e.g., a class 101 or a class 30injection molding machine, or an “ultra high productivity moldingmachine”), such as the high productivity injection molding machinedisclosed in U.S. patent application Ser. No. 13/601,514, filed Aug. 31,2012, which is hereby incorporated by reference herein, that may be usedto produce thinwalled consumer products, such as toothbrush handles andrazor handles. Thin walled parts are generally defined as having a highL/T ratio of 100 or more.

Referring to the figures in detail, FIG. 1 illustrates an exemplary lowconstant pressure injection molding apparatus 10 that generally includesan injection system 12 and a clamping system 14. A thermoplasticmaterial may be introduced to the injection system 12 in the form ofthermoplastic pellets 16. The thermoplastic pellets 16 may be placedinto a hopper 18, which feeds the thermoplastic pellets 16 into a heatedbarrel 20 of the injection system 12. The thermoplastic pellets 16,after being fed into the heated barrel 20, may be driven to the end ofthe heated barrel 20 by a reciprocating screw 22. The heating of theheated barrel 20 and the compression of the thermoplastic pellets 16 bythe reciprocating screw 22 causes the thermoplastic pellets 16 to melt,forming a molten thermoplastic material 24. The molten thermoplasticmaterial is typically processed at a temperature of about 130° C. toabout 410° C.

The reciprocating screw 22 forces the molten thermoplastic material 24,toward a nozzle 26 to form a shot of thermoplastic material, which willbe injected into a mold cavity 32 of a mold 28 via one or more gates 30,preferably three or less gates, that direct the flow of the moltenthermoplastic material 24 to the mold cavity 32. In other embodimentsthe nozzle 26 may be separated from one or more gates 30 by a feedsystem (not shown). The mold cavity 32 is formed between first andsecond mold sides 25, 27 of the mold 28 and the first and second moldsides 25, 27 are held together under pressure by a press or clampingunit 34. The press or clamping unit 34 applies a clamping force duringthe molding process that is greater than the force exerted by theinjection pressure acting to separate the two mold halves 25, 27,thereby holding the first and second mold sides 25, 27 together whilethe molten thermoplastic material 24 is injected into the mold cavity32. To support these clamping forces, the clamping system 14 may includea mold frame and a mold base. Once the shot of molten thermoplasticmaterial 24 is injected into the mold cavity 32, the reciprocating screw22 stops traveling forward. The molten thermoplastic material 24 takesthe form of the mold cavity 32 and the molten thermoplastic material 24cools inside the mold 28 until the thermoplastic material 24 solidifies.Once the thermoplastic material 24 has solidified, the press 34 releasesthe first and second mold sides 25, 27, the first and second mold sides25, 27 are separated from one another, and the finished part may beejected from the mold 28. The mold 28 may include a plurality of moldcavities 32 to increase overall production rates. The shapes of thecavities of the plurality of mold cavities may be identical, similar ordifferent from each other. (The latter may be considered a family ofmold cavities).

A controller 50 is communicatively connected with a nozzle sensor 52,located in the vicinity of the nozzle 26, an flow front sensor 53located within the mold cavity 32 or proximate the mold cavity 32, alinear transducer 57 located proximate the reciprocating screw 22, and ascrew control 36. The controller 50 may include a microprocessor, amemory, and one or more communication links. The flow front sensor 53may provide an indication of the location of a flow front of the moltenplastic material flowing through the mold cavity 32. While the flowfront sensor 53 is illustrated near an end of the mold cavity 32 (e.g.,the location in the mold cavity that is last to fill with molten plasticmaterial) in FIG. 1, the flow front sensor 53 may be located at anypoint in the mold cavity between a gate and the location of the moldcavity 32 that is last to fill with molten thermoplastic material. Ifthe flow front sensor 53 is not located near the end of the mold cavity32, a time correction factor may be applied to approximate when the flowfront of the molten plastic material will reach the end of the moldcavity 32. It may be desirable to locate the flow front sensor 53 within30% of an end of the mold cavity 32, preferably within 20% of the end ofthe mold cavity 32, and more preferably within 10% of the end of themold cavity.

The linear transducer 57 may measure an amount of linear movement of thereciprocating screw 22. The nozzle sensor 52 and the flow front sensor53 may sense the presence of thermoplastic material optically,pneumatically, electrically, ultrasonically, mechanically or otherwiseby sensing changes due to the arrival of the flow front of thethermoplastic material. The linear transducer may measure linearmovement mechanically, optically, pneumatically, magnetically,electrically, ultrasonically, or the linear transducer may use any othermethod of measuring linear movement. When pressure or temperature of thethermoplastic material is measured by the nozzle sensor 52, the nozzlesensor 52 may send a signal indicative of the pressure or thetemperature to the controller 50 to provide a target pressure for thecontroller 50 to maintain in the mold cavity 32 (or in the nozzle 26) asthe fill is completed. This signal may generally be used to control themolding process, such that variations in material viscosity, moldtemperatures, melt temperatures, and other variations influencingfilling rate, are adjusted by the controller 50. These adjustments maybe made immediately during the molding cycle, or corrections can be madein subsequent cycles. Furthermore, several signals may be averaged overa number of cycles and then used to make adjustments to the moldingprocess by the controller 50. The controller 50 may be connected to thenozzle sensor 52, the flow front sensor 53, the screw control 36, and/orthe linear transducer 56 via wired connections 54, 55, 56, 59,respectively. In other embodiments, the controller 50 may be connectedto the nozzle sensors 52, to the flow front sensor 53, to the screwcontrol 56, and to the linear transducer 57 via a wireless connection, amechanical connection, a hydraulic connection, a pneumatic connection,or any other type of wired or wireless communication connection known tothose having ordinary skill in the art that will allow the controller 50to communicate with the sensors 52, 53, 57 and/or to send a controlsignal to the screw control 36 or any other component of the injectionmolding machine.

In the embodiment of FIG. 1, the nozzle sensor 52 is a pressure sensorthat measures (directly or indirectly) melt pressure of the moltenthermoplastic material 24 in vicinity of the nozzle 26. The nozzlesensor 52 generates an electrical signal that is transmitted to thecontroller 50. The controller 50 then commands the screw control 36 toadvance the screw 22 at a rate that maintains a desired melt pressure ofthe molten thermoplastic material 24 in the nozzle 26. This is known asa pressure controlled process. While the nozzle sensor 52 may directlymeasure the melt pressure, the nozzle sensor 52 may also indirectlymeasure the melt pressure by measuring other characteristics of themolten thermoplastic material 24, such as temperature, viscosity, flowrate, etc, which are indicative of melt pressure. Likewise, the nozzlesensor 52 need not be located directly in the nozzle 26, but rather thenozzle sensor 52 may be located at any location within the injectionsystem 12 or mold 28 that is fluidly connected with the nozzle 26. Ifthe nozzle sensor 52 is not located within the nozzle 26, appropriatecorrection factors may be applied to the measured characteristic tocalculate an estimate of the melt pressure in the nozzle 26. The nozzlesensor 52 need not be in direct contact with the injected fluid and mayalternatively be in dynamic communication with the fluid and able tosense the pressure of the fluid and/or other fluid characteristics. Ifthe nozzle sensor 52 is not located within the nozzle 26, appropriatecorrection factors may be applied to the measured characteristic tocalculate the melt pressure in the nozzle 26. In yet other embodiments,the nozzle sensor 52 need not be disposed at a location that is fluidlyconnected with the nozzle. Rather, the nozzle sensor 52 could measureclamping force generated by the clamping system 14 at a mold partingline between the first and second mold parts 25, 27. In one aspect thecontroller 50 may maintain the pressure according to the input fromnozzle sensor 52. Alternatively, the sensor could measure an electricalpower demand by an electric press, which may be used to calculate anestimate of the pressure in the nozzle 26.

Although an active, closed loop controller 50 is illustrated in FIG. 1,other pressure regulating devices may be used instead of the closed loopcontroller 50. For example, a pressure regulating valve (not shown) or apressure relief valve (not shown) may replace the controller 50 toregulate the melt pressure of the molten thermoplastic material 24. Morespecifically, the pressure regulating valve and pressure relief valvecan prevent overpressurization of the mold 28. Another alternativemechanism for preventing overpressurization of the mold 28 is an alarmthat is activated when an overpressurization condition is detected.

Turning now to FIG. 2, an example molded part 100 is illustrated. Themolded part 100 is a thin-walled part. Molded parts are generallyconsidered to be thin-walled when a length of a flow channel L dividedby a thickness of the flow channel T is greater than 100 (i.e.,L/T>100), but less than 1000. For mold cavities having a morecomplicated geometry, the L/T ratio may be calculated by integrating theT dimension over the length of the mold cavity 32 from the gate 30 tothe end of the mold cavity 32, and determining the longest length offlow from the gate 30 to the end of the mold cavity 32. The L/T ratiocan then be determined by dividing the longest length of flow by theaverage part thickness. In the case where a mold cavity 32 has more thanone gate 30, the L/T ratio is determined by integrating L and T for theportion of the mold cavity 32 filled by each individual gate and theoverall L/T ratio for a given mold cavity is the highest L/T ratio thatis calculated for any of the gates. In some injection moldingindustries, thin-walled parts may be defined as parts having an L/T>100,or having an L/T>200, but <1000. The length of the flow channel L is thelongest flow length as measured from the gate 30 to the end 104 of themold cavity. Thin-walled parts are especially prevalent in the consumerproducts industry.

High L/T ratio parts are commonly found in molded parts having averagethicknesses less than about 10 mm. In consumer products, products havinghigh L/T ratios generally have an average thickness of less than about 5mm. For example, while automotive bumper panels having a high L/T ratiogenerally have an average thickness of 10 mm or less, tall drinkingglasses having a high L/T ratio generally have an average thickness ofabout 5 mm or less, containers (such as tubs or vials) having a high L/Tratio generally have an average thickness of about 3 mm or less, bottlecap enclosures having a high L/T ratio generally have an averagethickness of about 2 mm or less, and individual toothbrush bristleshaving a high L/T ratio generally have an average thickness of about 1mm or less. The low constant pressure injection molding processes anddevices disclosed herein are particularly advantageous for parts havinga thickness of 5 mm or less and the disclosed processes and devices aremore advantageous for thinner parts.

Thin-walled parts with high L/T ratios present certain obstacles ininjection molding. For example, the thinness of the flow channel tendsto cool the molten thermoplastic material before the material reachesthe flow channel end 104. When this happens, the thermoplastic materialfreezes off and no longer flows, which results in an incomplete part. Toovercome this problem, traditional injection molding machines inject themolten thermoplastic material at very high pressures, typically greaterthan 15,000 psi, so that the molten thermoplastic material rapidly fillsthe mold cavity before having a chance to cool and freeze off. This isone reason that manufacturers of the thermoplastic materials teachinjecting at very high pressures. Another reason traditional injectionmolding machines inject at high pressures is the increased shear, whichincreases flow characteristics, as discussed above. These very highinjection pressures require the use of very hard materials to form themold 28 and the feed system, among other things. Moreover, the thinwalled parts may include one or more special features 105, such as aliving hinge, a filament, a closure, a dispenser, a spout, a bellows,and an actuator, that must be filled before the material freezes.

When filling at a substantially constant pressure (during an injectionmolding cycle), it was generally thought that the filling rates wouldneed to be reduced relative to conventional filling methods. This meansthe polymer would be in contact with the cool molding surfaces forlonger periods before the mold would completely fill. Thus, more heatwould need to be removed before filling, and this would be expected toresult in the material freezing off before the mold is filled. It hasbeen unexpectedly discovered that the thermoplastic material will flowwhen subjected to substantially constant pressure conditions, during aninjection molding cycle, despite a portion of the mold cavity beingbelow the no-flow temperature of the thermoplastic material. It would begenerally expected by one of ordinary skill in the art that suchconditions would cause the thermoplastic material to freeze and plug themold cavity rather than continue to flow and fill the entire moldcavity. Without intending to be bound by theory, it is believed that thesubstantially constant pressure conditions, during an injection moldingcycle, of embodiments of the disclosed method and device allow fordynamic flow conditions (i.e., constantly moving melt front) throughoutthe entire mold cavity during filling. There is no hesitation in theflow of the molten thermoplastic material as it flows to fill the moldcavity and, thus, no opportunity for freeze-off of the flow despite atleast a portion of the mold cavity being below the no-flow temperatureof the thermoplastic material.

Additionally, it is believed that as a result of the dynamic flowconditions, the molten thermoplastic material is able to maintain atemperature higher than the no-flow temperature, despite being subjectedto such temperatures in the mold cavity, as a result of shear heating.It is further believed that the dynamic flow conditions interfere withthe formation of crystal structures in the thermoplastic material as itbegins the freezing process. Crystal structure formation increases theviscosity of the thermoplastic material, which can prevent suitable flowto fill the cavity. The reduction in crystal structure formation and/orcrystal structure size can allow for a decrease in the thermoplasticmaterial viscosity as it flows into the cavity and is subjected to thelow temperature of the mold that is below the no-flow temperature of thematerial.

The disclosed low constant pressure injection molding methods andsystems may use a sensor (such as the nozzle sensor 52, the flow frontsensor 53, or the linear transducer 57 in FIG. 1 above) located withinthe mold cavity or proximate the mold cavity to monitor changes inmaterial viscosity, changes in material temperature, and/or changes inother material properties. Measurements from these sensors may becommunicated to the controller 50 to allow the controller 50 to correctthe process in real time to ensure the melt front pressure is relievedprior to the melt front reaching the end of the mold cavity, which cancause flashing of the mold, and another pressure and power peak.Moreover, the controller 50 may use the sensor measurements to adjustthe peak power and peak flow rate points in the process, so as toachieve consistent processing conditions. In addition to using thesensor measurements to fine tune the process in real time during thecurrent injection cycle, the controller 50 may also to adjust theprocess over time (e.g., over a plurality of injection cycles). In thisway, the current injection cycle can be corrected based on measurementsoccurring during one or more cycles at an earlier point in time. In oneembodiment, sensor readings can be averaged over many cycles so as toachieve process consistency.

In various embodiments, the mold can include a cooling system thatmaintains the entire mold cavity at a temperature below the no-flowtemperature. For example, even surfaces of the mold cavity which contactthe shot comprising molten thermoplastic material can be cooled tomaintain a lower temperature. Any suitable cooling temperature can beused. For example, the mold can be maintained substantially at roomtemperature. Incorporation of such cooling systems can advantageouslyenhance the rate at which the as-formed injection molded part is cooledand ready for ejection from the mold.

Thermoplastic Material:

A variety of thermoplastic materials can be used in the low constantpressure injection molding methods and devices of the disclosure. In oneembodiment, the molten thermoplastic material has a viscosity, asdefined by the melt flow index of about 0.1 g/10 min to about 500 g/10min, as measured by ASTM D1238 performed at temperature of about 230 Cwith a 2.16 kg weight. For example, for polypropylene the melt flowindex can be in a range of about 0.5 g/10 min to about 200 g/10 min.Other suitable melt flow indexes include about 1 g/10 min to about 400g/10 min, about 10 g/10 min to about 300 g/10 min, about 20 to about 200g/10 min, about 30 g/10 min to about 100 g/10 min, about 50 g/10 min toabout 75 g/10 min, about 0.1 g/10 min to about 1 g/10 min, or about 1g/10 min to about 25 g/10 min. The MFI of the material is selected basedon the application and use of the molded article. For examples,thermoplastic materials with an MFI of 0.1 g/10 min to about 5 g/10 minmay be suitable for use as preforms for Injection Stretch Blow Molding(ISBM) applications. Thermoplastic materials with an MFI of 5 g/10 minto about 50 g/10 min may be suitable for use as caps and closures forpackaging articles. Thermoplastic materials with an MFI of 50 g/10 minto about 150 g/10 min may be suitable for use in the manufacture ofbuckets or tubs. Thermoplastic materials with an MFI of 150 g/10 min toabout 500 g/10 min may be suitable for molded articles that haveextremely high L/T ratios such as a thin plate. Manufacturers of suchthermoplastic materials generally teach that the materials should beinjection molded using melt pressures in excess of 6000 psi, and oftenin great excess of 6000 psi. Contrary to conventional teachingsregarding injection molding of such thermoplastic materials, embodimentsof the low constant pressure injection molding method and device of thedisclosure advantageously allow for forming quality injection moldedparts using such thermoplastic materials and processing at meltpressures below 15,000 psi, and possibly well below 15,000 psi.

The thermoplastic material can be, for example, a polyolefin. Exemplarypolyolefins include, but are not limited to, polypropylene,polyethylene, polymethylpentene, and polybutene-1. Any of theaforementioned polyolefins could be sourced from bio-based feedstocks,such as sugarcane or other agricultural products, to produce abio-polypropylene or bio-polyethylene. Polyolefins advantageouslydemonstrate shear thinning when in a molten state. Shear thinning is areduction in viscosity when the fluid is placed under compressivestress. Shear thinning can beneficially allow for the flow of thethermoplastic material to be maintained throughout the injection moldingprocess. Without intending to be bound by theory, it is believed thatthe shear thinning properties of a thermoplastic material, and inparticular polyolefins, results in less variation of the materialsviscosity when the material is processed at constant pressures. As aresult, embodiments of the method and device of the disclosure can beless sensitive to variations in the thermoplastic material, for example,resulting from colorants and other additives as well as processingconditions. This decreased sensitivity to batch-to-batch variations ofthe properties thermoplastic material can also advantageously allowpost-industrial and post consumer recycled plastics to be processedusing embodiments of the method and the device of the disclosure.Post-industrial, post consumer recycled plastics are derived from endproducts that have completed their life cycle as a consumer item andwould otherwise have been disposed of as a solid waste product. Suchrecycled plastic, and blends of thermoplastic materials, inherently havesignificant batch-to-batch variation of their material properties.

The thermoplastic material can also be, for example, a polyester.Exemplary polyesters include, but are not limited to, polyethyleneterphthalate (PET). The PET polymer could be sourced from bio-basedfeedstocks, such as sugarcane or other agricultural products, to producea partially or fully bio-PET polymer. Other suitable thermoplasticmaterials include copolymers of polypropylene and polyethylene, andpolymers and copolymers of thermoplastic elastomers, polyester,polystyrene, polycarbonate, poly(acrylonitrile-butadiene-styrene),poly(lactic acid), bio-based polyesters such as poly(ethylene furanate)polyhydroxyalkanoate, poly(ethylene furanoate), (considered to be analternative to, or drop-in replacement for, PET), polyhydroxyalkanoate,polyamides, polyacetals, ethylene-alpha olefin rubbers, andstyrene-butadiene-styrene block copolymers. The thermoplastic materialcan also be a blend of multiple polymeric and non-polymeric materials.The thermoplastic material can be, for example, a blend of high, medium,and low molecular polymers yielding a multi-modal or bi-modal blend. Themulti-modal material can be designed in a way that results in athermoplastic material that has superior flow properties yet hassatisfactory chemo/physical properties. The thermoplastic material canalso be a blend of a polymer with one or more small molecule additives.The small molecule could be, for example, a siloxane or otherlubricating molecule that, when added to the thermoplastic material,improves the flowability of the polymeric material.

Other additives may include inorganic fillers such calcium carbonate,calcium sulfate, talcs, clays (e.g., nanoclays), aluminum hydroxide,CaSiO3, glass formed into fibers or microspheres, crystalline silicas(e.g., quartz, novacite, crystallobite), magnesium hydroxide, mica,sodium sulfate, lithopone, magnesium carbonate, iron oxide; or, organicfillers such as rice husks, straw, hemp fiber, wood flour, or wood,bamboo or sugarcane fiber.

Other suitable thermoplastic materials include renewable polymers suchas nonlimiting examples of polymers produced directly from organisms,such as polyhydroxyalkanoates (e.g., poly(beta-hydroxyalkanoate),poly(3-hydroxybutyrate-co-3-hydroxyvalerate, NODAX (RegisteredTrademark)), and bacterial cellulose; polymers extracted from plants,agricultural and forest, and biomass, such as polysaccharides andderivatives thereof (e.g., gums, cellulose, cellulose esters, chitin,chitosan, starch, chemically modified starch, particles of celluloseacetate), proteins (e.g., zein, whey, gluten, collagen), lipids,lignins, and natural rubber; thermoplastic starch produced from starchor chemically starch and current polymers derived from naturally sourcedmonomers and derivatives, such as bio-polyethylene, bio-polypropylene,polytrimethylene terephthalate, polylactic acid, NYLON 11, alkyd resins,succinic acid-based polyesters, and bio-polyethylene terephthalate. Thesuitable thermoplastic materials may include a blend or blends ofdifferent thermoplastic materials such in the examples cited above. Aswell the different materials may be a combination of materials derivedfrom virgin bio-derived or petroleum-derived materials, or recycledmaterials of bio-derived or petroleum-derived materials. One or more ofthe thermoplastic materials in a blend may be biodegradable. And fornon-blend thermoplastic materials that material may be biodegradable.

Exemplary thermoplastic resins together with their recommended operatingpressure ranges are provided in the following table:

Injection Pressure Range Material Material Full Name (PSI) Company BrandName pp Polypropylene 10000- RTP RTP 100 15000 Imagineering seriesPlastics Poly- propylene Nylon 10000- RTP RTP 200 18000 Imagineeringseries Nylon Plastics ABS Acrylonitrile 8000- Marplex Astalac ABSButadiene 20000 Styrene PET Polyester 5800- Asia AIE PET 14500International 401F Acetal 7000- API Kolon Kocetal Copolymer 17000 PCPolycarbonate 10000- RTP RTP 300 15000 Imagineering series PlasticsPoly- carbonate PS Polystyrene 10000- RTP RTP 400 15000 Imagineeringseries Plastics SAN Styrene 10000- RTP RTP 500 Acrylonitrile 15000Imagineering series Plastics PE LDPE & HDPE 10000- RTP RTP 700 15000Imagineering Series Plastics TPE Thermoplastic 10000- RTP RTP 1500Elastomer 15000 Imagineering series Plastics PVDF Polyvinylidene 10000-RTP RTP 3300 Fluoride 15000 Imagineering series Plastics PTIPolytrimethylene 10000- RTP RTP 4700 Terephthalate 15000 Imagineeringseries Plastics PBT Polybutylene 10000- RTP RTP 1000 Terephthalate 15000Imagineering series Plastics PLA Polylactic Acid 8000- RTP RTP 209915000 Imagineering series Plastics

While more than one of the embodiments involves filling substantiallythe entire mold cavity with the shot comprising the molten thermoplasticmaterial while maintaining the melt pressure of the shot comprising themolten thermoplastic material at a substantially constant pressure,during the injection molding cycle, specific thermoplastic materialsbenefit from the invention at different constant pressures.Specifically: PP, nylon, PC, PS, SAN, PE, TPE, PVDF, PTI, PBT, and PLAat a substantially constant pressure of less than 10000 psi; ABS at asubstantially constant pressure of less than 8000 psi; PET at asubstantially constant pressure of less than 5800 psi; Acetal copolymerat a substantially constant pressure of less than 7000 psi; pluspoly(ethylene furanate) polyhydroxyalkanoate, polyethylene furanoate(aka PEF) at substantially constant pressure of less than 10000 psi, or8000 psi, or 7000 psi or 6000 psi, or 5800 psi.

As described in detail above, embodiments of the disclosed low constantpressure injection molding method and device can achieve one or moreadvantages over conventional injection molding processes. For example,embodiments include a more cost effective and efficient process thateliminates the need to balance the pre-injection pressures of the moldcavity and the thermoplastic materials, a process that allows for use ofatmospheric mold cavity pressures and, thus, simplified mold structuresthat eliminate the necessity of pressurizing means, the ability to uselower hardness, high thermal conductivity mold cavity materials that aremore cost effective and easier to machine, a more robust processingmethod that is less sensitive to variations in the temperature,viscosity, and other material properties of the thermoplastic material,and the ability to produce quality injection molded parts atsubstantially constant pressures without premature hardening of thethermoplastic material in the mold cavity and without the need to heator maintain constant temperatures in the mold cavity.

Turning now to FIG. 3, a typical pressure-time curve for a conventionalhigh variable pressure injection molding process is illustrated by thedashed line 200. By contrast, a pressure-time curve for the disclosedlow constant pressure injection molding machine is illustrated by thesolid line 210. In the conventional case, melt pressure is rapidlyincreased to well over 15,000 psi and then held at a relatively highpressure, more than 15,000 psi, for a first period of time 220. Thefirst period of time 220 is the fill time in which molten plasticmaterial flows into the mold cavity. Thereafter, the melt pressure isdecreased and held at a lower, but still relatively high pressure,typically 10,000 psi or more, for a second period of time 230. Thesecond period of time 230 is a packing time in which the melt pressureis maintained to ensure that all gaps in the mold cavity are backfilled. After packing is complete, the pressure may optionally bedropped again for a third period of time 232, which is the cooling time.The mold cavity in a conventional high variable pressure injectionmolding system is packed from the end of the flow channel back totowards the gate. The material in the mold typically freezes off nearthe end of the cavity, then completely frozen off region of materialprogressively moves toward the gate location, or locations. As a result,the plastic near the end of the mold cavity is packed for a shorter timeperiod and with reduced pressure, than the plastic material that iscloser to the gate location, or locations. Part geometry, such as verythin cross sectional areas midway between the gate and end of moldcavity, can also influence the level of packing pressure in regions ofthe mold cavity. Inconsistent packing pressure may cause inconsistenciesin the finished product, as discussed above. Moreover, the conventionalpacking of plastic in various stages of solidification results in somenon-ideal material properties, for example, molded-in stresses, sink,and non-optimal optical properties.

The low constant pressure injection molding system, on the other hand,injects the molten plastic material into the mold cavity at asubstantially constant pressure for a fill time period 240. Theinjection pressure in the example of FIG. 3 is less than 6,000 psi.However, other embodiments may use higher pressures. After the moldcavity is filled, the low constant pressure injection molding systemgradually reduces pressure over a second time period 242 as the moldedpart is cooled. By using a substantially constant pressure during theinjection molding cycle, the molten thermoplastic material maintains acontinuous melt flow front that advances through the flow channel fromthe gate towards the end of the flow channel. In other words, the moltenthermoplastic material remains moving throughout the mold cavity, whichprevents premature freeze off. Thus, the plastic material remainsrelatively uniform at any point along the flow channel, which results ina more uniform and consistent finished product. By filling the mold witha relatively uniform pressure, the finished molded parts formcrystalline structures that may have better mechanical and opticalproperties than conventionally molded parts. Moreover, the parts moldedat constant pressures exhibit different characteristics than skin layersof conventionally molded parts. As a result, parts molded under constantpressure may have better optical properties than parts of conventionallymolded parts. Turning now to FIG. 4, the various stages of fill arebroken down as percentages of overall fill time. For example, in anconventional high variable pressure injection molding process, the fillperiod 220 makes up about 10% of the total fill time, the packing period230 makes up about 50% of the total fill time, and the cooing period 232makes up about 40% of the total fill time. On the other hand, in the lowconstant pressure injection molding process, the fill period 240 makesup about 90% of the total fill time while the cooling period 242 makesup only about 10% of the total fill time. The low constant pressureinjection molding process needs less cooling time because the moltenplastic material is cooling as it is flowing into the mold cavity. Thus,by the time the mold cavity is filled, the molten plastic material hascooled significantly, although not quite enough to freeze off in thecenter cross section of the mold cavity, and there is less total heat toremove to complete the freezing process. Additionally, because themolten plastic material remains liquid throughout the fill, and packingpressure is transferred through this molten center cross section, themolten plastic material remains in contact with the mold cavity walls(as opposed to freezing off and shrinking away). As a result, the lowconstant pressure injection molding process described herein is capableof filling and cooling a molded part in less total time than in aconventional injection molding process.

In the disclosed low constant pressure injection molding method anddevice for molding a high L/T part, the part is molded by injecting amolten thermoplastic polymer into a mold cavity at an increasing flowrate to achieve a desired injection pressure and then decreasing theflow rate over time to maintain a substantially constant injectionpressure. The low constant pressure injection molding method and deviceare particularly advantageous when molding thinwall parts (e.g., partshaving an L/T ratio>100<1000) and when using shot sizes of between 0.1 gand 100 g. It is especially advantageous that the maximum flow rateoccur within the first 30% of cavity fill, preferably within the first20% of cavity fill, and even more preferably within the first 10% ofcavity fill. By adjusting the filling pressure profile the maximum flowrate occurs within these preferred ranges of cavity fill, the moldedpart will have at least some of the physical advantages described above(e.g., better strength, better optical properties, etc.) because thecrystalline structure of the molded part is different from aconventionally molded part. Moreover, because high L/T products arethinner, these products require less pigment to impart a desired colorto the resulting product. Furthermore, in no-pigment parts, the partswill have less visible deformities due to the more consistent moldingconditions. Using less or no pigment saves costs.

Alternatively, the peak power may be adjusted to maintain asubstantially constant injection pressure. More specifically, thefilling pressure profile may be adjusted to cause the peak power tooccur in the first 30% of the cavity fill, preferably in the first 20%of the cavity fill, and even more preferably in the first 10% of thecavity fill. Adjusting the process to cause the peak power to occurwithin the preferred ranges, and then to have a decreasing powerthroughout the remainder of the cavity fill results in the same benefitsfor the molded part that were described above with respect to adjustingpeak flow rate. Moreover, adjusting the process in the manner describedabove is particularly advantageous for thinwall parts (e.g., L/Tratio>100<1000) and for shot sizes of between 0.1 g and 100 g).

Turning now to FIGS. 5A-5D and FIGS. 6A-6D a portion of a mold cavity asit is being filled by a conventional injection molding machine (FIGS.5A-5D) and as it is being filled by a substantially constant pressureinjection molding machine (FIGS. 6A-6D) is illustrated.

As illustrated in FIGS. 5A-5D, as the conventional injection moldingmachine begins to inject molten thermoplastic material 24 into a moldcavity 32 through the gate 30, the high injection pressure tends toinject the molten thermoplastic material 24 into the mold cavity 32 at ahigh rate of speed, which causes the molten thermoplastic material 24 toflow in laminates 31, most commonly referred to as laminar flow (FIG.5A). These outermost laminates 31 adhere to walls of the mold cavity andsubsequently cool and freeze, forming a frozen boundary layer 33 (FIG.5B), before the mold cavity 32 is completely full. As the thermoplasticmaterial freezes, however, it also shrinks away from the wall of themold cavity 32, leaving a gap 35 between the mold cavity wall and theboundary layer 33. This gap 35 reduces cooling efficiency of the mold.Molten thermoplastic material 24 also begins to cool and freeze in thevicinity of the gate 30, which reduces the effective cross-sectionalarea of the gate 30. In order to maintain a constant volumetric flowrate, the conventional injection molding machine must increase pressureto force molten thermoplastic material through the narrowing gate 30. Asthe thermoplastic material 24 continues to flow into the mold cavity 32,the boundary layer 33 grows thicker (FIG. 5C). Eventually, the entiremold cavity 32 is substantially filled by thermoplastic material that isfrozen (FIG. 5D). At this point, the conventional injection moldingmachine must maintain a packing pressure to push the receded boundarylayer 33 back against the mold cavity 32 walls to increase cooling.

A low constant pressure injection molding machine, on the other hand,flows molten thermoplastic material into a mold cavity 32 with aconstantly moving flow front 37 (FIGS. 6A-6D). The thermoplasticmaterial 24 behind the flow front 37 remains molten until the moldcavity 37 is substantially filled (i.e., 99% or more filled) beforefreezing. As a result, there is no reduction in effectivecross-sectional area of the gate 30, which may be between 70% and 100%,preferably between 80% and 90%, of the nominal wall thickness of themolded part. Moreover, because the thermoplastic material 24 is moltenbehind the flow front 37, the thermoplastic material 24 remains incontact with the walls of the mold cavity 32. As a result, thethermoplastic material 24 is cooling (without freezing) during the fillportion of the molding process. Thus, the cooling portion of thedisclosed low constant pressure injection molding process need not be aslong as a conventional process.

Because the thermoplastic material remains molten and keeps moving intothe mold cavity 32, less injection pressure is required than inconventional molds. In one embodiment, the injection pressure may be6,000 psi or less. As a result, the injection systems and clampingsystems need not be as powerful. For example, the disclosed low constantpressure injection molding devices may use clamps requiring lowerclamping forces, and a corresponding lower clamping power source.Moreover, the disclosed low constant pressure injection moldingmachines, because of the lower power requirements, may employ electricpresses, which are generally not powerful enough to use in conventionalclass 101 and 102 injection molding machines that mold thinwall parts athigh variable pressures. Even when electric presses are sufficient touse for some simple, molds with few mold cavities, the process may beimproved with the disclosed low constant pressure injection moldingmethods and devices as smaller, less expensive electric motors may beused. The disclosed low constant pressure injection molding machines maycomprise one or more of the following types of electric presses, adirect servo drive motor press, a dual motor belt driven press, a dualmotor planetary gear press, and a dual motor ball drive press having apower rating of 200 HP or less.

Turning now to FIG. 7, operation of an example molding cycle 1000 forthe low constant pressure injection molding process is illustrated. Themolding cycle 1000 may be carried out on a low constant pressureinjection molding machine constructed in accordance with the disclosure,for example, on the low constant pressure injection molding machine ofFIG. 1. More specifically, the example molding cycle 1000 may be carriedout on a low constant pressure injection molding machine having a moldincluding a first mold side and a second mold side, at least one of thefirst mold side and the second mold side having an average thermalconductivity of more than 51.9 W/m-° C. (30 BTU/HR FT ° F.) and lessthan or equal to 385.79 W/m-° C. (223 BTU/HR FT ° F.), and a mold cavitythat is formed between the first mold side and the second mold side. Insome preferred embodiments, both the first and second mold side may havean average thermal conductivity of more than 51.9 W/m-° C. (30 BTU/HR FT° F.) and less than or equal to 385.79 W/m-° C. (223 BTU/HR FT ° F.).

Some preferred materials for manufacturing the first and/or second moldsides include aluminum (for example, 2024 aluminum, 2090 aluminum, 2124aluminum, 2195 aluminum, 2219 aluminum, 2324 aluminum, 2618 aluminum,5052 aluminum, 5059 aluminum, aircraft grade aluminum, 6000 seriesaluminum, 6013 aluminum, 6056 aluminum, 6061 aluminum, 6063 aluminum,7000 series aluminum, 7050 aluminum, 7055 aluminum, 7068 aluminum, 7075aluminum, 7076 aluminum, 7150 aluminum, 7475 aluminum, QC-10, Alumold™,Hokotol™, Duramold 2™, Duramold 5™, and Alumec 99™), BeCu (for example,C17200, C 18000, C61900, C62500, C64700, C82500, Moldmax LH™, MoldmaxHH™, and Protherm™), Copper, and any alloys of aluminum (e.g.,Beryllium, Bismuth, Chromium, Copper, Gallium, Iron, Lead, Magnesium,Manganese, Silicon, Titanium, Vanadium, Zinc, Zirconium), any alloys ofcopper (e.g., Magnesium, Zinc, Nickel, Silicon, Chromium, Aluminum,Bronze). These materials may have Rockwell C (Rc) hardnesses of between0.5 Rc and 20 Rc, preferably between 2 Rc and 20 Rc, more preferablybetween 3 Rc and 15 Rc, and more preferably between 4 Rc and 10 Rc.While these materials may be softer than tool steels, the thermalconductivity properties are more desirable. The disclosed low constantpressure injection molding methods and devices advantageously operateunder molding conditions that allow molds made of these softer, higherthermal conductivity, materials to extract useful lives of more than 1million cycles, preferably between 1.25 million cycles and 10 millioncycles, and more preferably between 2 million cycles and 5 millioncycles.

Initially, molten thermoplastic material is advanced into a mold cavitythat defines a thin-walled part (e.g., 100<L/T<1000) at 1110. A shot ofmolten thermoplastic material may be between 0.5 g and 100 g and may beadvanced through three or fewer gates into the mold cavity. In somecases one or more of the three of fewer gates may have a cross-sectionalarea that is between 70% and 100% of a nominal wall thickness of a partthat is formed in the mold cavity, and preferably between 80% and 90% ofthe nominal wall thickness. In some examples, this percentage maycorrespond to a gate size of between 0.5 mm and 10 mm.

Molten thermoplastic material is advanced into the mold cavity until themold cavity is substantially filled at 1112. The mold cavity may besubstantially filled when the mold cavity is more than 90% filled,preferably more than 95% filled and more preferably more than 99%filled. After the mold cavity is substantially filled, the moltenthermoplastic material is cooled at 1114 until the molten thermoplasticmaterial is substantially frozen or solidified. The molten thermoplasticmaterial may be actively cooled with a cooling liquid flowing through atleast one of the first and second mold sides, or passively cooledthrough convection and conduction to the atmosphere.

After the thermoplastic material is cooled, the first and second moldsides may be separated to expose the cooled thermoplastic material at1116. The cooled thermoplastic material (in the form of the molded part)may be removed from the mold at 1118. The thermoplastic material may beremoved by, for example, ejection, dumping, extraction (manually or viaan automated process), pulling, pushing, gravity, or any other method ofseparating the cooled thermoplastic material from the first and secondmold sides.

After the cooled thermoplastic material is removed from the first andsecond mold sides, the first and second mold sides may be closed,reforming the mold cavity, at 1120, which prepares the first and secondmold sides to receive a new shot of molten thermoplastic material,thereby completing a single mold cycle. Cycle time 1001 is defined as asingle iteration of the molding cycle 1000. A single molding cycle maytake between 2 seconds and 15 seconds, preferably between 8 seconds and10 seconds, depending on the part size and material.

All injection molding processes are susceptible to variations in theviscosity of the molten plastic material. Variations in the viscosity ofthe molten plastic material may cause imperfections in the molded part,such as insufficient material (e.g., short shot), and flashing. Anynumber of factors can cause the viscosity of the molten plastic materialto vary. For example, changes in ambient temperature or pressure, theaddition of a colorant, changes in shear conditions between the feedsystem and the last cavity location to fill with molten plastic material(otherwise known as the “end of fill location”), viscosity variations inthe virgin plastic material itself, and changes in other conditions allmay cause the viscosity of the molten plastic material to change. Asviscosity of the molten plastic material changes, pressure required toforce the molten plastic into the mold will also change. For example, ifviscosity increases, pressure required to force the polymer into themold cavity will increase because the polymer is thicker and harder tomove into the mold cavity. On the other hand, as viscosity decreases,pressure required force the polymer into the mold cavity will decreasebecause the polymer is thinner and easier to move into the mold cavity.If no adjustments are made to the injection pressure or the cycle time,the molded part may have flaws. Current injection molding machines andprocesses have molding cycles that are time-based. In other words, themolding cycle is controlled by time, among other factors, as theinjection molding cycle is ended at a predetermined time. As a result,changes in viscosity to the molten plastic material may cause the moltenplastic material to reach in end of the mold cavity at a time that isdifferent from the predetermined time.

Turning now to FIG. 8, a pressure versus time graph is illustrated for asingle injection molding cycle. During an initial phase of the injectionmolding cycle pressure rapidly increases to a predetermined target value1210 (e.g., a “fill pressure”), where the pressure is held as the moldcavity is filled. When molten plastic material nears the end of the moldcavity 32, as indicated by the flow front sensor 53 (FIG. 1), at a firsttime t_(t) (or t_(transducer)) 1212, pressure is reduced at 1214 to alower pressure (e.g., a “pack and hold pressure”) as the material in themold cavity 32 cools. At a second time t_(s) (or t_(step)) 1216, whichis a total cycle time from initiation of the filling sequence to an endof the filling cycle where the mold is opened in the molded part isejected from the mold cavity 32.

Changes in viscosity of the molten plastic material may affect the timeat which the molten plastic material reaches the end of the mold cavity32 or the end of fill location in the mold cavity at t_(s). For example,if viscosity of the molten plastic material increases, (with thepossibility of a “short shot”), then the molten plastic material may bemaintained at the fill pressure for a longer time, as illustrated bydashed line 1220 a. In this example, the flow front sensor 53 may detectthe molten plastic material at a time that is later than a predeterminedtime. A predetermined time for the molten plastic to reach the flowfront sensor may be calculated or derived experimentally for idealconditions and constant viscosity for the molten plastic material. Onthe other hand, if viscosity of the molten plastic material decreases,(with the possibility of “flashing”), then the molten plastic materialmay be maintained at the fill pressure for a shorter time, asillustrated by the dashed line 1220 b. In this example, the flow frontsensor 53 may detect molten plastic material at a time t_(t) that isearlier than the predetermined time.

In the case of a pressure controlled filling process, an optimal fillingpressure profile can be determined experimentally. This optimal pressureprofile establishes the optimal plastic pressures for a nominal polymerat each instant of the filling cycle. Polymer pressure is sensed at alocation in the polymer flow channel prior to the mold cavity. In oneexample, polymer pressure may be sensed by the nozzle sensor 52.However, the polymer pressure may be sensed at any location upstream ofthe gate. A nominal flow rate may be assigned to each instant during thefilling cycle by calculating or directly measuring the polymer flow rateduring the filling cycle. Furthermore, force required to move a givenvolume of the polymer through the feed system and in to the mold cavitymay be determined for the optimal filling pressure profile throughexperimentation or calculation.

For each filling cycle, the controller 50 may compare the nominal (oroptimal) force required to move polymer through the system and theactual force required to move a given volume of polymer through the feedsystem during the filling cycle. If the actual force is higher than thenominal force, then material flowability has increased. As a result, acorresponding and proportional increase to the filling force is requiredto generate the optimal polymer flow rate profile.

Polymer pressure, at any given point in the system, is an indication ofa magnitude of force that is required to move polymer through thesystem. Additionally, polymer pressure, at any given point in thesystem, is directly proportional to a magnitude of force that isrequired to move polymer through the system. As a result, polymerpressure may be used to balance or offset shrinkage forces in thepolymer as it cools in the mold cavity.

For a hydraulic injection molding press, a proportional valve may beused to regulate the flow of hydraulic fluid to the system acting uponthe injection screw 22. An electronic control signal may be sent to theproportional valve, which causes the proportional valve to move inresponse to the control signal. When the voltage of the control signalto the proportional valve increases, the proportional valve moves todeliver more hydraulic pressure to the reciprocating screw 22, therebyforcing more polymer through the feed system. As-polymer viscositychanges, the resistance to flow through the system will change, asdiscussed above. Thus, to maintain the polymer flow rate through thefeed system at a rate that is equivalent to the flow rate for a nominalfluid viscosity material, the voltage of the control signal to theproportional valve must be changed. This same relationship may beestablished for electric presses, where the voltage of the controlsignal changes amperage to a servo motor. Or for electric-hydraulichybrid injection presses where separate, but synchronized, controlsignals are sent to electric servo drives and to a proportional valve.

Turning now to FIG. 9A, a graph 1310 illustrates control signal response(in voltage) from the controller to the proportional valve over time.The vertical axis of the graph 1312 corresponds to control signalvoltage in milliamps and the horizontal axis of the graph 1314corresponds to time in milliseconds. A ratio of the change in thevoltage of the control signal to the proportional valve to the timeperiod for the change may be used to control the injection process. Thisvalue can be expressed as a ratio of millivolts to time, for example,and is defined herein as the Flow Factor (FF). The Flow Factor ratio maybe determined for an injection molding process for any molding machine,including hydraulic, electric, or any other molding machine. The FlowFactor (FF) may be mathematically represented by the following formula:

FF=(CS1−CS2)T;

wherein CS1 and CS2 are control signals (that may be measured, forexample, in millivolts) that are measured at a distinct moments in timeduring the injection molding cycle.

While reference is made herein to a “first control signal” and a “secondcontrol signal,” the control signal from the controller 50 to theproportional valve is a continuous signal, as illustrated in FIG. 9A.However, this control signal may be measured at distinct moments in time(e.g., a first time t1 1316 and a second time t2 1318) during theinjection molding cycle to define the first and second control signals.The voltages measured at these distinct moments in time are referred toherein as the first control signal voltage 1320 and the second controlsignal voltage 1322, respectively. While the control signals in thedescribed embodiments are defined, at least in part, by electricalvoltages, other control signal parameters may be used similarly. Forexample if the control signal were pneumatic in nature, the flow factormay be defined by the control signal parameter of pressure. Similarly,if the control signal were optical in nature, the flow factor may bedefined by the control signal parameter of brightness or intensity.Those skilled in the art may select an appropriate control signalparameter given the nature of the control signal itself.

Preferred time increments for t1 and t2 may lie between 0.1 millisecondsand 10 milliseconds, preferably between 0.5 milliseconds and 5milliseconds, more preferably between 0.75 milliseconds and 2milliseconds, and even more preferably about 1 millisecond. Time periodsin the disclosed ranges provide a very responsive control system thatquickly adjusts the control signal in response to changes inflowability.

In the case of a pressure controlled process, such as the pressurecontrolled processes described herein, the controller 50 may adjust theflow of polymer material to reach a predetermined pressure set point. Asflowability decreases, and corresponding resistances along the flowlength drop, more flow is required to reach the pressure set point, andthus the actual FF₁ may be higher than the expected or optimal FF. Thisrelationship may be used to detect, at any point, instantaneously orperiodically, in an injection molding cycle, whether the systemflowability has changed. This system flowability shift could be causedby any number of factors, including molecular weight distributionchanges in the polymer, and shear or temperature changes. For example,if the actual FF is higher than the optimal FF, then the flowability hasdecreased. Similarly, if the actual FF is lower than the optimal FF,then flowability has increased. This change in FF is caused by thesystem attempting to maintain a predetermined pressure. If theflowability is lowered, the proportional valve must release additionalhydraulic pressure to the system to force the higher viscosity materialto reach the pre-determined pressure set point. Likewise, if theflowability is increased, then the proportional valve must release lesshydraulic pressure to the system to force the lower viscosity materialto reach the pre-determined pressure set point. The control response toincrease or decrease polymer pressure could be linear or non-linear andmay be based on dependent or independent system variables.

Furthermore, the difference between the actual FF and the optimal FF maybe calibrated to the nominal viscosity of the polymer, such that achange the control signal is calibrated to a corresponding change in theflowabilty of molten plastic material, as measured in real time. Thus,this difference between the actual FF and the optimal FF, may be indexedrelative to flowability. The actual FF may be compared to the optimal FFat any point in the process, and the difference may be used to adjust atarget filling pressures for the remainder of the filling cycle. Forexample, if the FF is compared early in the cycle (e.g., the first 10%of the cycle), then the target pressure for the remainder of the cycle(e.g., the last 90%) may be adjusted upward or downward based on thecomparison between the actual FF and the optimal FF. Moreover, thecomparison between the actual FF and the optimal FF may be used toadjust the target pressure of a subsequent injection cycle, even ifadjustment to the target pressure within the current injection cycle ismade. By comparing the actual FF to the optimal FF, the target fillingpressure may be adjusted intra cycle or inter cycle to account forchanges in polymer flowability.

In some instances, a factor may be defined that incorporates thedifference between first and second control signals as a ratio to theincremental movement of the injection unit (for example, as measured bythe linear transducer 57 and representing a volume of material flow perunit of movement). Such a factor is further defined herein as aViscosity Change Index (VCI). VCI takes in to account the influence ofchanges in the system Pressure (P), changes to polymer Volume (V),system Temperature (T), and material composition or molecular weightdistribution (C). VCI is a function of the sum of the influences of P,V, T, and C in the total polymer flow system, and is represented by theformula:

VCI=f[ΔP,ΔV,ΔT,ΔC].

Or expressed as a measured variable:

VCI=(CS1−CS2)/S;

where CS1 is a first control signal, CS2 is a second control signal andS is a positional difference for a melt moving machine component, suchas the injection screw 22;VCI may be used to control a process to compensate for changes in theflowability of the molten plastic material during an injection moldingcycle of the injection molding system, thereby enabling instantaneousadjustments to the filling process to achieve the optimal polymer flowrate profile. Similar to FF above, changes in VCI indicate changes inflowability, as measured in real time, at any point in the filling cycleand thereafter may be used to increase or decrease filling pressure inresponse to the change.]

Turning now to FIG. 9B, a graph 1350 illustrates control signal response(in voltage) from the controller to the proportional valve over time.The vertical axis of the graph 1352 corresponds to control signalvoltage in milliamps and the horizontal axis of the graph 1354corresponds to a distance movement of the melt moving machine componentin microns.

The control signal may be measured at distinct moments in time (e.g., afirst time t1 1356 and a second time t2 1358) during the injectionmolding cycle. The voltages measured at these distinct moments in timeare referred to herein as the first control signal voltage 1360 and thesecond control signal voltage 1362, respectively. While the controlsignals in the described embodiments are defined, at least in part, byelectrical voltages, other control signal parameters may be usedsimilarly. For example if the control signal were pneumatic in nature,the flow factor may be defined by the control signal parameter ofpressure. Similarly, if the control signal were optical in nature, theflow factor may be defined by the control signal parameter of brightnessor intensity. Those skilled in the art may select an appropriate controlsignal parameter given the nature of the control signal itself. VCI actsas a “soft sensor” in the system, since it uses other sensors andmeasured variables to calculate a value corresponding to changes inpolymer viscosity. FF may also be used as a “soft sensor” in the system.The VCI “soft sensor” may be used to determine changes in materialflowability during the filling cycle and thus to enable instantaneousadjustments to the filling process to achieve the optimal polymer flowrate profile. Changes in VCI may indicate changes in the flowability ofthe molten plastic material at any point in the filling cycle, and thenthis value may be used to control filling pressure. VCI may bedetermined at a plurality of instantaneous points during the fillingcycle, by comparing the ratio of the difference between CV1 and CV2, asa ratio to a 1 micron travel of the injection unit. If VCI hasincreased, then a corresponding control response is made to increase thetarget filling pressure for the next increment of fill to compensate forthe shift in flowability. If VCI has decreased, then a correspondingcontrol response is made to decrease the target filling pressure for thenext increment of fill. The control response may be linear or non-linearand may be adjusted based on dependent or independent system variables.

For the purposes of controlling the injection molding process, it ispossible to calculate VCI at any point in an injection molding cycle,and then to use VCI as a basis for a single adjustment to the targetfilling pressure for subsequent portions of the injection molding cycle.If VCI is calculated early in the injection molding cycle (e.g., thefirst 10% of the cycle), then the target filling pressure for theremainder of the injection molding cycle (e.g., the remaining 90% of thecycle) may be adjusted upward or downward based on the VCI calculation.Alternatively, the VCI calculation may be used to adjust a subsequentinjection molding cycle, even if a target pressure of the currentinjection molding cycle is not adjusted. By calculating a VCI a controlresponse may be made to increase or decrease filling pressure in realtime. By calculating VCI (or FF) in real time, the disclosed system andmethod advantageously react nearly instantaneously to changes in systemflowability and also adjust process variables quickly to compensate forany flowability changes for a current injection cycle as well as forsubsequent injection cycles. The disclosed soft sensors may beincorporated into control logic that determines corrective inputs basedon rates of change of the soft sensors, or differences between the softsensor values and reference curves or data tables.

Injection pressure may also be adjusted on a continuous basis throughoutthe injection molding cycle. For example, when VCI is calculated at anypoint in the injection molding cycle the VCI may be fed forward and anadjustment in the target filling pressure may be made for the nextincrement of the injection molding cycle. For example, VCI may becalculated every 0.5 milliseconds and subsequent target fillingpressures may be adjusted based on the immediately previous VCI (or anaverage of one or more previous VCIs). This process may be repeatedthroughout the injection molding cycle, thus controlling the process ona closed loop basis throughout the injection molding cycle. Whencalculating VCI a control response may be made to increase or decreasein filling pressure.

Individual press specifications, such as proportional valve design,servo motor power output, barrel diameter, hydraulic capacity, checkring performance, and other press variables may be accounted for in thecalculation of the control variables by using ratios or by usingdimensionless factors. The flowability of a polymer material may varyduring an injection molding process for several reasons. A nominalviscosity of the polymer may vary slightly from batch-to-batch due toinconsistent manufacturing or poor quality control. When running amolding operation, these viscosity variations result in a change in thepressures required to fill the mold cavity. These viscosity variationscan result in quality issues, and reduced productivity. For example, ifthe viscosity increases substantially, the injection molding system maynot produce adequate force to push the polymer into the mold, thusresulting in a “short shot”. Furthermore, parts can become trapped inthe mold if the short shot region prevents the part from being properlyejected from the mold, and the trapped part can damage the mold uponclosing of the mold in preparation for the next shot of polymer. This isespecially concerning in the case of a soft metallurgy mold, such asaluminum, where the material is more easily deformed than hardermaterials, such as hardened tools steels. If the polymer viscositydecreases substantially, too much pressure may be applied to fill themold resulting in “flashing” the mold. When flashing occurs a partingline edge can become damaged as material is forced between shut offsurfaces. Flashing is especially concerning in the case of a softmetallurgy mold, such as aluminum, where the material is more easilydeformed than harder materials, such as hardened tools steels.

In the case of recycled polymers, the feedstock is highly variable dueto the multiple sources of material that are blended together to form abatch of recycled resin. The recycler seeks to blend many different meltflow index (MFI) resins together to achieve a target average MFI.However, the materials comprising the blend are highly variable, andthus the resulting blend can range as much as plus or minus 10 MFI, oreven greater. For the processor, this creates a difficult processingscenario, since the MFI variation causes constant changes in processingconditions. In this case, the machine operator must regularly monitorthe process and resulting parts and make regular corrections to theprocess to avoid producing scrap parts or damaging the mold.

The addition of colorant masterbatch may also result in a change in theviscosity of the polymer. These viscosity variations require the moldingmachine operator to make processing adjustments to account for thesevariations. Furthermore, the masterbatch is concentrated and added at arelatively low levels typically ranging from about 0.5 to about 10% ofthe total material composition. Small variations in the addition of themasterbatch, which is inherent in the metering methods of masterbatchaddition, can result in large swings in polymer viscosity. As a result,the operator must tend to the molding machine to make needed adjustmentsto avoid producing scrap parts or damaging the mold.

Viscosity is also temperature dependent. In a molding operation, thetemperature of the water cooling a molding system may vary. For example,when using tower water to cool a mold the temperature of the water willbe higher on a warmer day, especially after continued warm weather, andwill be cooler in the evening or during prolonged cooler weather. Thesetemperature variations may cause the viscosity of the material in themolding system to change. As a result, part quality varies and createsthe potential for scrap and damage to the mold. Other sources oftemperature variation may include temperature variations that occur inthe nozzle, or in the heater bands maintaining an elevated temperaturein the feed system. Molding equipment operators must continually monitorand adjust for these temperature swings.

In order to correct the problems caused by changes in flowability, thecontroller 50 (FIG. 1) may cause the screw control 26 (FIG. 1) toincrease or decrease movement of the reciprocating screw 22 based on thechange in flowability to maintain a target injection pressure.

Turning now to FIG. 10, the logic diagram 1400 of the process foraccounting for changes in viscosity is illustrated. An injection moldingmachine with at least one mold cavity is provided at 1410. an injectionmolding controller is provided at 1420, which includes a pressurecontrol output that is configured to provide a control signal, which, atleast partially, determines an injection molding pressure for theinjection molding process of the injection molding machine. A firstcontrol signal is measured at 1430 at a first time in the injectionmolding cycle. A second control signal is measured at 1440 at a secondtime in the injection molding cycle, subsequent to the first time. Thefirst control signal from the pressure control output and the secondcontrol signal from the pressure control output are compared at 1450. Athird control signal for the pressure control output is determined at1460 at a third time, subsequent to the second time, the third controlsignal being based at least in part on the comparison result.

FIG. 11 illustrates one embodiment of a closed loop control system thatmay be used to carry out the method illustrated in FIG. 10. The controlsystem 1700 illustrated in FIG. 11 provides an open architecture forthose skilled in the art to implement and refine the injection moldingprocess illustrated in FIG. 10 for particular parts or materials byusing any one or more the process variables represented generally byreference numeral 1710. These process variables 1710 represent physicalvalues obtained from various sensors in the injection molding machine.These physical values are processed by the controller 50. The processingmay include any number of calculations such as the calculationsindicated by reference numeral 1712. These calculations may be used forconversion or scaling in order to reflect the importance of particularvariables through weighting. Such calculations may incorporate rulebased algorithms, heuristic algorithms, genetic algorithms, or othercalculations, such as the calculations described above with respect flowfactor and viscosity change index.

The process variables 1710 may be used directly, with or withoutscaling, or via variable calculations 1712, to just a core variable1714, and to determine a control variable 1716, such as a controlsignal. For example, the variable calculations 1712 may include the VCIor flow factor calculations 1713 described above. The control variable1716 represents a feedback signal for the PID control loop 1718. Whileany number of desired setpoints, generally represented by referencenumeral 1720, may be used as an input to the PID control loop 1718, thesystems and methods described herein, which are pressure controlinjection molding processes, generally use changes in injection pressure1722 as a control input. As a result, the PID control loop 1718continuously adjusts a control signal that is sent to the proportionalvalve 1724, thereby continuously accounting for changes in theflowability of the molten plastic material.

In some cases, the injection molding machine may include an electricpress and the controller may vary an electronic control signal to aservo motor of the electric press.

As discussed above, changes in the viscosity of the molten plasticmaterial may be caused by any number of factors. For example, anoperator may desire to reuse poor quality parts by re-grinding the poorquality parts and mixing the reground plastic material with virginplastic material. Mixing of regrind and virgin plastic material willchange the MFI of the combined material. Similarly, an operator maydesire to change part color during an injection run by introducing acolorant into the molten plastic material. The introduction of acolorant will often change the MFI of the molten plastic material.Finally, changes in ambient operating conditions can also change theviscosity of the molten plastic material. For example, if ambienttemperature increases, viscosity of the molten plastic material oftenincreases. Likewise, if ambient temperature decreases, viscosity themolten plastic material often decreases.

The disclosed low constant pressure injection molding methods andmachines advantageously reduce cycle time for the molding process whileincreasing part quality. Moreover, the disclosed low constant pressureinjection molding machines may employ, in some embodiments, electricpresses, which are generally more energy efficient and require lessmaintenance than hydraulic presses. Additionally, the disclosed lowconstant pressure injection molding machines are capable of employingmore flexible support structures and more adaptable delivery structures,such as wider platen widths, increased tie bar spacing, elimination oftie bars, lighter weight construction to facilitate faster movements,and non-naturally balanced feed systems. Thus, the disclosed lowconstant pressure injection molding machines may be modified to fitdelivery needs and are more easily customizable for particular moldedparts.

Additionally, the disclosed low constant pressure injection moldingmachines and methods allow the molds to be made from softer materials(e.g., materials having a Rc of less than about 30), which may havehigher thermal conductivities (e.g., thermal conductivities greater thanabout 20 BTU/HR FT ° F.), which leads to molds with improved coolingcapabilities and more uniform cooling. Because of the improved coolingcapabilities, the disclosed low constant pressure injection molds mayinclude simplified cooling systems. Generally speaking, the simplifiedcooling systems include fewer cooling channels and the cooling channelsthat are included may be straighter, having fewer machining axes. Oneexample of an injection mold having a simplified cooling system isdisclosed in U.S. Patent Application No. 61/602,781, filed Feb. 24,2012, which is hereby incorporated by reference herein.

The lower injection pressures of the low constant pressure injectionmolding machines allow molds made of these softer materials to extract 1million or more molding cycles, which would not be possible inconventional injection molding machines as these materials would failbefore 1 million molding cycles in a high pressure injection moldingmachine.

It is noted that the terms “substantially,” “about,” and“approximately,” unless otherwise specified, may be utilized herein torepresent the inherent degree of uncertainty that may be attributed toany quantitative comparison, value, measurement, or otherrepresentation. These terms are also utilized herein to represent thedegree by which a quantitative representation may vary from a statedreference without resulting in a change in the basic function of thesubject matter at issue. Unless otherwise defined herein, the terms“substantially,” “about,” and “approximately” mean the quantitativecomparison, value, measurement, or other representation may fall within20% of the stated reference.

It should now be apparent that the various embodiments of the productsillustrated and described herein may be produced by a low, substantiallyconstant pressure molding process. While particular reference has beenmade herein to products for containing consumer goods or consumer goodsproducts themselves, it should be apparent that the molding methoddiscussed herein may be suitable for use in conjunction with productsfor use in the consumer goods industry, the food service industry, thetransportation industry, the medical industry, the toy industry, and thelike. Moreover, one skilled in the art will recognize the teachingsdisclosed herein may be used in the construction of stack molds,multiple material molds including rotational and core back molds, incombination with in-mold decoration, insert molding, in mold assembly,and the like.

Part, parts, or all of any of the embodiments disclosed herein can becombined with part, parts, or all of other injection molding embodimentsknown in the art, including those described below.

Embodiments of the present disclosure can be used with embodiments forinjection molding at low constant pressure, as disclosed in U.S. patentapplication Ser. No. 13/476,045 filed May 21, 2012, entitled “Apparatusand Method for Injection Molding at Low Constant Pressure” (applicant'scase 12127) and published as US 2012-0294963 A1, which is herebyincorporated by reference.

Embodiments of the present disclosure can be used with embodiments forpressure control, as disclosed in U.S. patent application Ser. No.13/476,047 filed May 21, 2012, entitled “Alternative Pressure Controlfor a Low Constant Pressure Injection Molding Apparatus” (applicant'scase 12128) now U.S. Pat. No. 8,757,999, which is hereby incorporated byreference.

Embodiments of the present disclosure can be used with embodiments fornon-naturally balanced feed systems, as disclosed in U.S. patentapplication Ser. No. 13/476,073 filed May 21, 2012, entitled“Non-Naturally Balanced Feed System for an Injection Molding Apparatus”(applicant's case 12130) and published as US 2012-0292823 A1, which ishereby incorporated by reference.

Embodiments of the present disclosure can be used with embodiments forinjection molding at low, substantially constant pressure, as disclosedin U.S. patent application Ser. No. 13/476,197 filed May 21, 2012,entitled “Method for Injection Molding at Low, Substantially ConstantPressure” (applicant's case 12131Q) and published as US 2012-0295050 A1,which is hereby incorporated by reference.

Embodiments of the present disclosure can be used with embodiments forinjection molding at low, substantially constant pressure, as disclosedin U.S. patent application Ser. No. 13/476,178 filed May 21, 2012,entitled “Method for Injection Molding at Low, Substantially ConstantPressure” (applicant's case 12132Q) and published as US 2012-0295049 A1,which is hereby incorporated by reference.

Embodiments of the present disclosure can be used with embodiments forco-injection processes, as disclosed in U.S. patent application Ser. No.13/774,692 filed Feb. 22, 2013, entitled “High Thermal ConductivityCo-Injection Molding System” (applicant's case 12361), which is herebyincorporated by reference.

Embodiments of the present disclosure can be used with embodiments formolding with simplified cooling systems, as disclosed in U.S. patentapplication Ser. No. 13/765,428 filed Feb. 12, 2013, entitled “InjectionMold Having a Simplified Evaporative Cooling System or a SimplifiedCooling System with Exotic Cooling Fluids” (applicant's case 12453M),now U.S. Pat. No. 8,591,219, which is hereby incorporated by reference.

Embodiments of the present disclosure can be used with embodiments formolding thinwall parts, as disclosed in U.S. patent application Ser. No.13/476,584 filed May 21, 2012, entitled “Method and Apparatus forSubstantially Constant Pressure Injection Molding of Thinwall Parts”(applicant's case 12487), which is hereby incorporated by reference.

Embodiments of the present disclosure can be used with embodiments formolding with a failsafe mechanism, as disclosed in U.S. patentapplication Ser. No. 13/672,246 filed Nov. 8, 2012, entitled “InjectionMold With Fail Safe Pressure Mechanism” (applicant's case 12657), whichis hereby incorporated by reference.

Embodiments of the present disclosure can be used with embodiments forhigh-productivity molding, as disclosed in U.S. patent application Ser.No. 13/682,456 filed Nov. 20, 2012, entitled “Method for Operating aHigh Productivity Injection Molding Machine” (applicant's case 12673R),which is hereby incorporated by reference.

Embodiments of the present disclosure can be used with embodiments formolding certain thermoplastics, as disclosed in U.S. patent applicationSer. No. 14/085,515 filed Nov. 20, 2013, entitled “Methods of MoldingCompositions of Thermoplastic Polymer and Hydrogenated Castor Oil”(applicant's case 12674M), which is hereby incorporated by reference.

Embodiments of the present disclosure can be used with embodiments forrunner systems, as disclosed in U.S. patent application Ser. No.14/085,515 filed Nov. 21, 2013, entitled “Reduced Size Runner for anInjection Mold System” (applicant's case 12677M), which is herebyincorporated by reference.

Embodiments of the present disclosure can be used with embodiments formoving molding systems, as disclosed in U.S. patent application61/822,661 filed May 13, 2013, entitled “Low Constant Pressure InjectionMolding System with Variable Position Molding Cavities:” (applicant'scase 12896P), which is hereby incorporated by reference.

Embodiments of the present disclosure can be used with embodiments forinjection mold control systems, as disclosed in U.S. patent application61/861,298 filed Aug. 20, 2013, entitled “Injection Molding Machines andMethods for Accounting for Changes in Material Properties DuringInjection Molding Runs” (applicant's case 13020P), which is herebyincorporated by reference.

Embodiments of the present disclosure can be used with embodiments forinjection mold control systems, as disclosed in U.S. patent application61/861,304 filed Aug. 20, 2013, entitled “Injection Molding Machines andMethods for Accounting for Changes in Material Properties DuringInjection Molding Runs” (applicant's case 13021P), which is herebyincorporated by reference.

Embodiments of the present disclosure can be used with embodiments forinjection mold control systems, as disclosed in U.S. patent application61/861,310 filed Aug. 20, 2013, entitled “Injection Molding Machines andMethods for Accounting for Changes in Material Properties DuringInjection Molding Runs” (applicant's case 13022P), which is herebyincorporated by reference.

Embodiments of the present disclosure can be used with embodiments forusing injection molding to form overmolded articles, as disclosed inU.S. patent application 61/918,438 filed Dec. 19, 2013, entitled“Methods of Forming Overmolded Articles” (applicant's case 13190P),which is hereby incorporated by reference.

Embodiments of the present disclosure can be used with embodiments forcontrolling molding processes, as disclosed in U.S. Pat. No. 5,728,329issued Mar. 17, 1998, entitled “Method and Apparatus for Injecting aMolten Material into a Mold Cavity” (applicant's case 12467CC), which ishereby incorporated by reference.

Embodiments of the present disclosure can be used with embodiments forcontrolling molding processes, as disclosed in U.S. Pat. No. 5,716,561issued Feb. 10, 1998, entitled “Injection Control System” (applicant'scase 12467CR), which is hereby incorporated by reference.

Embodiments of the present disclosure can be used with embodiments formolding preforms, as disclosed in U.S. patent application 61/952,281,entitled “Plastic Article Forming Apparatus and Methods for Using theSame” (applicant's case 13242P), which is hereby incorporated byreference.

Embodiments of the present disclosure can be used with embodiments formolding preforms, as disclosed in U.S. patent application 61/952,283,entitled “Plastic Article Forming Apparatus and Methods for Using theSame” (applicant's case 13243P), which is hereby incorporated byreference. The dimensions and values disclosed herein are not to beunderstood as being strictly limited to the exact numerical valuesrecited. Instead, unless otherwise specified, each such dimension isintended to mean both the recited value and a functionally equivalentrange surrounding that value. For example, a dimension disclosed as “40mm” is intended to mean “about 40 mm.”

Every document cited herein, including any cross referenced or relatedpatent or application and any patent application or patent to which thisapplication claims priority or benefit thereof, is hereby incorporatedherein by reference in its entirety unless expressly excluded orotherwise limited. The citation of any document is not an admission thatit is prior art with respect to any invention disclosed or claimedherein or that it alone, or in any combination with any other referenceor references, teaches, suggests or discloses any such invention.Further, to the extent that any meaning or definition of a term in thisdocument conflicts with any meaning or definition of the same term in adocument incorporated by reference, the meaning or definition assignedto that term in this document shall govern.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope of this invention.

What is claimed is:
 1. A method of automatically adjusting an injectionmolding process to compensate for variations in the flowability of amolten plastic material, the method comprising: providing an injectionmolding machine with at least one mold cavity; providing an injectionmolding controller, which includes a pressure control output that isconfigured to provide a control signal, which, at least partiallydetermines an injection molding pressure for the injection moldingprocess of the injection molding machine; measuring a first controlsignal from the pressure control output at a first time in an injectionmolding cycle; measuring a second control signal from the pressurecontrol output at a second time in the injection molding cycle,subsequent to the first time; comparing the first control signal fromthe pressure control output and the second control signal from thepressure control output to obtain a comparison result; and determining athird control signal for the pressure control output, based at least inpart on the comparison result, at a third time that is subsequent to thesecond time.
 2. The method of claim 1, wherein the determining includesdetermining the third control signal at a third time, which is withinthe same injection molding cycle as the second time.
 3. The method ofclaim 1, wherein the third time is located in a subsequent molding cyclefrom the second time.
 4. The method of claim 1, including: determining atime difference between the first time and the second time; and whereinthe comparing includes comparing the first control signal and the secondcontrol signal, based, at least in part, on the time difference, toobtain the comparison result.
 5. The method of claim 4, wherein thecomparison result is a flow factor (FF) that is used as a soft sensormelt viscosity input to by the controller.
 6. The method of claim 5,wherein the FF is determined by the formula:FF=(CS1−CS2)/T; where CS1 is the first control signal; CS2 is the secondcontrol signal; and T is the time difference between CS1 and CS2.
 7. Themethod of claim 6, wherein the third control signal is proportional tothe flow factor.
 8. The method of claim 6, wherein T is between 0.1milliseconds and 10 milliseconds.
 9. The method of claim 6, wherein T isabout 1 millisecond.
 10. The method of claim 1, wherein the comparisonresult is used as a basis for a viscosity change index (VCI) that isused as a soft sensor melt viscosity input to by the controller.
 11. Themethod of claim 10, wherein the VCI is determined by the followingformula:VCI=(CS1−CS2)/S where CS1 is a first control signal; CS2 is a secondcontrol signal; and S is the position difference for the a melt movingmachine component.
 12. The method of claim 11, wherein the third controlsignal is proportional to the VCI.
 13. The method of claim 11, wherein Sis between 0.5 microns and 10 microns.
 14. The method of claim 11,wherein S is about 1 micron.
 15. The method of claim 11, wherein thefirst control signal and the second control signal are measured beforethe molten plastic material fills more than 90% of a mold cavity. 16.The method of claim 1, wherein the comparing of the first control signaland the second control signal includes comparing the first controlsignal and the second control signal to optimal control signals based onan optimal pressure curve.
 17. The method of claim 1, wherein theproviding of the injection molding machine includes providing a meltmoving machine component; and further comprising: measuring a firstposition of the melt moving machine component at the first time;measuring a second position of the melt moving machine component at thesecond time; determining a position difference between the firstposition and the second position; and wherein the comparing includescomparing the first control signal and the second control signal, based,at least in part, on the position difference, to obtain the comparisonresult.
 18. The method of claim 1, further comprising controlling theinjection molding pressure by sending the third control signal to a meltpressure control device.
 19. A controller configured to perform themethod of claim
 1. 20. A molding machine that includes the controller ofclaim 19.